While you can stop the credit expiring by purchasing more credit, you can’t buy a large initial bundle and then top up with small amounts effectively, since the rate per megabyte is much higher on the cheaper plans.

Tesler: So the hardware people made it as big as they could in the address space, and then after some testing of the system we determined that half a megabyte was a reasonable compromise of cost and performance.

The new system, by employing the Ku satellite frequency band, will give Panasonic a fixed yearly expense for transponder rentals; they won't be paying for satellite time by the minute or megabyte, which is how Inmarsat charges its customers.